Highly active mixed-metal catalysts made by pulsed-laser ablation in liquids

ABSTRACT

The invention is directed to mixed-metal nanocatalysts, particularly nano-dimensioned layered double-hydroxide nanostacks, methods of making nanocatalysts using laser ablation techniques, and the electrochemical devices comprising and using these nanocatalysts, for example in the electrochemical oxidation of water oxidation.

CROSS REFERENCE TO RELATED APPLICATIONS

This applications claims priority to U.S. Patent Application No. 62/013,976, filed Jun. 18, 2014, the contents of which are incorporated by reference in their entirety herein.

GOVERNMENT RIGHTS

This invention was made with government support under CHE1305124 awarded by the National Science Foundation. The government has certain rights in the invention.

TECHNICAL FIELD

The invention relates to mixed-metal nanocatalysts, particularly nano-dimensioned layered double-hydroxide nanostacks, methods of making nanocatalysts using laser ablation techniques, and the electrochemical devices comprising and using these nanocatalysts, for example in the electrochemical oxidation of water oxidation.

BACKGROUND

Conversion of solar energy into storable fuels in a clean and sustainable way will be essential to meet future global energy demand. Worldwide scalability requires materials to be made from earth-abundant elements, Splitting water into oxygen and hydrogen with only sunlight as energy input is seen as a particularly attractive route. Rut such systems for the production of solar fuels will require robust, highly active catalysts.

Most widely used water oxidation catalysts are based on rare metals such as ruthenium and iridium. First-row transition metal oxides and hydroxides continue to attract attention because of their low cost and stability in base. The overpotentials of earth-abundant catalysts at 10 mA cm⁻² typically range from 350 to 430 mV in pH 14 aqueous electrolytes. Researchers have shown that hollow spheres of α-Ni(OH)₂ catalyzed water oxidation in base with an overpotential of 331 mV at 10 mA cm⁻² on glassy carbon working electrodes. But such overpotentials are unsuitably high for large scale applicability of such systems.

SUMMARY

The present invention is directed to nanocatalysts and novel methods of making and using such nanocatalysts that overcome some of the deficiencies of the prior art. Certain embodiments of the present invention provide nanocatalysts, especially layered double hydroxide nanostacks comprising a plurality of nanosheets represented by the general formula: [M_((1-x))M′_(x) (OH)₂]^(x+), wherein: M is a metal cation in a formal +2 oxidation state; M′ a metal cation in a formal +3 oxidation state; wherein

the nanosheets are associated with or intercalate [A^(m−) _(x/m)], where A is a displaceable anion;

m is an integer;

x is a positive number less than 1 (preferably, 0.5 or less);

the nanostack being optionally hydrated with a stoichiometric amount or a non-stoichiometric amount of water; and

the nanosheet having at least one lateral edge dimension in a range of from about 5 nm to about 500 nm.

In some of these embodiments, M is at least one of Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Sr²⁺, or Zn²⁺, and M′ is at least one of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, or Y³⁺. In certain of these embodiments, M is or comprises Ni²⁺ and/or M′ is or comprises Fe³⁺.

In some of these embodiments, A independently comprises one or more organic or inorganic anion, for example F⁻, Cl⁻, Br⁻, I⁻, BF₄ ⁻, PF₆ ⁻, CO₃ ²⁻, HCO₃ ⁻, CrO₄ ²⁻, NO₂ ⁻, NO₃ ⁻, ONO₂ ⁻, ClO₂ ⁻, ClO₃ ⁻, ClO₄.⁻, H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, IO₃ ⁻, OH⁻, S²⁻, SO₃ ²⁻, S₂O₃ ²⁻, SO₄ ²⁻, WO₄ ²⁻, acetate, propionate, lactate, terephthalate, adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a combination thereof. In some preferred embodiments, A is or comprises hydroxide.

In other embodiments, x is in a range of from 0.05 to 0.95.

The nanocatalysts are nanodimensioned and where present as layered double hydroxide nanostacks comprising a plurality of nanosheets, nanosheets can have at least one lateral edge dimension in a range of from about 5 nm to about 50 nm, for example from about 7 nm to about 22 nm.

Where the nanocatalysts are present as layered double hydroxide nanostacks, at least one of the nanosheets may be further doped with at least one Lewis acid ion of a transition metal, lanthanide, or actinide metal ion, for example doped with titanium or lanthanum.

Additional embodiments include those of methods of subjecting a solid ablation target to an energy source, the solid ablation target comprising first metal capable of oxidizing to a positive oxidation state, the energy source impinging on the first metal in the presence of a solution, preferably an aqueous solution, containing a second metal ion in a positive oxidation state for a time and energy sufficient to ionize at least a portion of the first metal to a positive oxidation state. When the solution is aqueous, the methods provide for the inventive layered double-hydroxide nanostacks described herein. Preferably, these aqueous ablation solutions are substantially free of surfactants. The energy for the ablation may be applied using a laser, for example, a pulsed laser, the pulsed laser optionally capable of delivering or delivering energy in a range of from about 90 mJ/pulse to about 210 mJ/pulse.

The nanocatalysts described herein may also be incorporated into electronic or electrochemical devices (e.g., electrochemical cells or sensors). Some embodiments provide for electrodes having coating comprising the nanocatalysts. In some of these embodiments, the layered double hydroxide nanostacks described herein are water oxidation catalysts. Associated embodiments include those where the inventive electrodes are used to pass sufficient current to oxidize water to form oxygen. These electrodes may comprise gold, nickel, platinum, or an allotrope of carbon, such as graphite, graphene, glassy (or vitreous) carbon, diamond, or a combination thereof. These nanostacks coatings may allow for the electrodes to exhibit water oxidation overpotentials of less than 300 mV at 10 mA/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

The present application is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the subject matter, exemplary embodiments of the subject matter are shown in the drawings; however, the presently disclosed subject matter is not limited to the specific methods, devices, and systems disclosed. In addition, the drawings are not necessarily drawn to scale. In the drawings:

FIGS. 1A and 1B provide schematic structural representations of the [Ni—Fe]-LDH [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O.

FIG. 2 shows overpotential η for water oxidation at 10 mA cm⁻² vs. Ni content for catalysts 1 to 5. Depicted in the photos are catalysts 3 to 5 in aqueous suspension to visualize their different colors.

FIG. 3 shows XRD data of catalyst 5 after anodization, (a) on Si, (b) on carbon cloth after 30 min anodization in 1.0 M pH 14.0 aqueous KOH at 0.807 V vs NHE, (c) on carbon cloth before anodization; bare carbon cloth (d).

FIG. 4A shows Tafel plots of current density (j) as a function of electrode polarization potential (E_(p)) (circles, 5; squares, 6; gray squares, Ni oxide electrodeposited according to Dinec{hacek over (a)}, M.; Surendranath, Y; Nocera, D. G Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 10337; gray circles, bare electrode), and a photograph of 5 and 6; FIG. 4B shows XRD data of catalysts 5 (bottom trace) and 6 (top trace), where *[Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O, |NiFe₂O₄ spinel; FIG. 4C shows far-IR spectra of catalysts 5 (bottom trace) and catalyst 6 (top trace).

FIG. 5A shows TOF vs. N_(405.1 eV)/N_(407.3 eV) (for neat Fe—Ni-based catalysts; and catalysts, 7 and 8). FIG. 5B shows XPS data of catalysts 4 to 8 (the vertical dashed lines mark the N 1s binding energies (405.1 and 407.3 eV)).

FIG. 6 shows determination of R_(u) for post-measurement iR drop correction; circles, measured data; line, linear fit.

FIG. 7 shows XPS data of catalysts 1 to 8 in the Fe 2p, Ni 2p, O 1s, and N 1s regions. The gray dashed lines are at the N 1s binding energies of interstitial (405.1 eV) and surface-adsorbed (407.3 eV) nitrate.

FIG. 8 shows XPS data of catalysts 7 and 8 in the Ti 2p and La 3d regions.

FIG. 9 shows XRD data of catalysts 1 to 8. Normalized fixed slit intensities of known minerals are displayed as vertical lines: black, maghemite; cyan, magnetite; red, jamborite; gray, goethite; blue, trevorite; green, TiO₂; purple, ulvospinel; yellow, Ni₃TiO₅; dark blue, La(Ni,Fe)O₃.

FIG. 10 shows TEM images of water oxidation catalysts 1 to 8. The insets show particles that imaged with a higher contrast. All scale bars are 20 nm.

FIG. 11 shows BET data of catalysts 5 to 8; P/P₀ denotes the relative pressure, and W is the weight of the adsorbed argon,

FIG. 12 shows Raman spectra of catalysts 1 to 8 (black). The sharp spikes in the spectrum of 2 are from cosmic ray events. Also depicted is a reference spectrum of the α-Ni(OH)₂ mineral jamborite (red, RRUFF ID R070619, collected with 532 nm excitation).

FIG. 13 shows infrared spectra of catalysts 5 (bottom trace) and 6 (top trace) with band assignments. The inset shows a magnification of the adsorbed and interstitial nitrate (ν3) region: open circles, data; thick lines, overall fits; thin lines Gaussian peak fits. The band was best fit by two Gaussian distributions, indicating the presence of two distinct nitrate species.

FIG. 14 shows infrared spectra (solid lines) of catalysts 5 and 6 with spectral deconvolutions (dotted lines).

FIG. 15 shows cyclic voltammograms of catalysts 1 to 8; j, current density, E_(p), polarization potential. The disjointed segments in the measured data occurred due to bubble release from the electrode surface.

FIG. 16 shows Tafel data of catalysts 1 to 8 (marked black squares); j, current density, Ep, polarization potential. For comparison, Tafel data of electrodeposited nickel oxide (unlabeled gray squares, same mass loading as catalysts) and bare HOPG (gray circles) are also plotted. The solid lines are fits. Plotting the overpotential at 10 mA cm⁻² vs. the Ni content in the catalyst (from XPS data) shows that the highest water oxidation activity was obtained with the highest Ni content (78%) in the material.

FIG. 17 shows current density j as a function of time data of catalysts 5, 6 and 8.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is directed to nanocatalysts, especially layered double-hydroxide nanostacks, methods employing novel laser ablation techniques to make such nanocatalysts, and methods and devices for using the inventive nanocatalysts.

The present invention may be understood more readily by reference to the following description taken in connection with the accompanying Figures and Examples, all of which form a part of this disclosure. It is to be understood that this invention is not limited to the specific products, methods, conditions or parameters described or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of any claimed invention. Similarly, unless specifically otherwise stated, any description as to a possible mechanism or mode of action or reason for improvement is meant to be illustrative only, and the invention herein is not to be constrained by the correctness or incorrectness of any such suggested mechanism or mode of action or reason for improvement. Throughout this text, it is recognized that the descriptions refer to compositions and methods of making and using said compositions. That is, where the disclosure describes or claims a feature or embodiment associated with a composition or a method of making or using a composition, it is appreciated that such a description or claim is intended to extend these features or embodiment to embodiments in each of these contexts (i.e., compositions, methods of making, and methods of using).

In the present disclosure the singular forms “a,” “an,” and “the” include the plural reference, and reference to a particular numerical value includes at least that particular value, unless the context clearly indicates otherwise. Thus, for example, a reference to “a material” is a reference to at least one of such materials and equivalents thereof known to those skilled in the art, and so forth.

When a value is expressed as an approximation by use of the descriptor “about,” it will be understood that the particular value forms another embodiment. In general, use of the term “about” indicates approximations that can vary depending on the desired properties sought to be obtained by the disclosed subject matter and is to be interpreted in the specific context in which it is used, based on its function. The person skilled in the art will be able to interpret this as a matter of routine. In some cases, the number of significant figures used for a particular value may be one non-limiting method of determining the extent of the word “about.” In other cases, the gradations used in a series of values may be used to determine the intended range available to the term “about” for each value. Where present, all ranges are inclusive and combinable. That is, references to values stated in ranges include every value within that range.

It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. That is, unless obviously incompatible or specifically excluded, each individual embodiment is deemed to be combinable with any other embodiment(s) and such a combination is considered to be another embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any sub-combination. Finally, while an embodiment may be described as part of a series of steps or part of a more general structure, each said step may also be considered an independent embodiment in itself, combinable with others.

The transitional terms “comprising,” “consisting essentially of,” and “consisting” are intended to connote their generally in accepted meanings in the patent vernacular; that is, (i) “comprising,” which is synonymous with “including,” “containing,” or “characterized by,” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps; (ii) “consisting of” excludes any element, step, or ingredient not specified in the claim; and (iii) “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention. Embodiments described in terms of the phrase “comprising” (or its equivalents), also provide, as embodiments, those which are independently described in terms of “consisting of” and “consisting essentially of” For those embodiments provided in terms of “consisting essentially of,” the basic and novel characteristic(s) is the facile operability of the methods (and the systems used in such methods and the compositions derived therefrom) to prepare and use the inventive materials, and the materials themselves, where the methods and materials are capable of delivering the highlighted properties using only the elements provided in the claims. That is, while other materials may also be present in the inventive compositions, the presence of these extra materials is not necessary to provide the described benefits of those compositions or devices (i.e., the effects may be additive) and/or these additional materials do not compromise the performance of the product compositions or devices. Similarly, where additional steps may also be employed in the methods, their presence is not necessary to achieve the described effects or benefits and/or they do not compromise the stated effect or benefit.

When a list is presented, unless stated otherwise, it is to be understood that each individual element of that list, and every combination of that list, is a separate embodiment. For example, a list of embodiments presented as “A, B, or C” is to be interpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A or C,” “B or C,” or “A, B, or C.”

Throughout this specification, words are to be afforded their normal meaning, as would be understood by those skilled in the relevant art. However, so as to avoid misunderstanding, the meanings of certain terms will be specifically defined or clarified.

For example, while generally understood by those skilled in the electrochemical arts, the term “overpotential” refers to a potential more positive than the thermodynamic onset voltage for oxygen evolution from water at a positive electrode and more negative than the thermodynamic onset voltage for hydrogen evolution from water at a negative electrode.

The term “plurality” connote a number of two or more.

Layered double hydroxides (LDHs), which belong to a typical family of anionic layered materials, can generally be represented by the general formula [M_(1-x)M′_(x)(OH)₂]^(x+)(A^(n−))_(x/n). mH₂O, where M is a bivalent metal cation, M′ is a trivalent metal cation, A^(n−) denotes an interlayer anions with negative charge n, m is the number of interlayer water molecules, and x is the molar ratio of the M′ to the sum of the M and M′. The term “double,” then, in this context refers to the presence of two oxidation states, not to the presence of only two metals. The identity and ratio of the layer elements as well as the interlayer guest anions can be adjusted over a wide range in order to obtain materials with specific structures and properties. Because of their flexible composition and versatility, LDHs have been widely investigated for their potential applications in the fields of catalysis, adsorption, ion exchange, cosmetics, biocidal and drug delivery, and functional materials.

LDHs are traditionally synthesized by coprecipitation methods, hydrothermal methods, ion-exchange methods or calcination-rehydration methods. A large amount of sodium salt is produced as by-product during the preparation of LDHs by traditional methods. The sodium salt mother liquor is always discharged directly due to the high energy costs of evaporation, and thus leads to environmental pollution. In addition, the use of strong alkali in the synthesis process means that the product must be well washed with water (tens or even hundred times of the product's mass), which leads to significant waste of water and problems with treatment of the alkaline effluent. Thus it is necessary to develop an environmentally friendly technology for preparation of LDHs.

The coprecipitation method is the most popular method used to prepare LDHs. A mixed salt solution containing the metal ions which constitute the layers are coprecipitated with alkali in order to obtain the LDHs. Either the mixed salt solution or the alkali solution can contain the corresponding interlayer anionic group. Additionally, LDHs may be prepared by coprecipitation of a mixture of soluble salts of bivalent and trivalent metal ions with Na₂CO₃ and NaOH. However, in this method, large quantity of water is required to wash the product after the reaction, due to the large amount of sodium salts produced in the reaction and the strongly alkaline solution, and a significant waste of water is thus caused.

The hydrothermal method for preparation of LDHs is another method in which insoluble oxides or hydroxides containing the metal ions to be incorporated in the layers are treated with water at a high temperature under a high pressure. In this method, Na₂CO₃ or NaHCO₃ may generally used as main starting materials, and the sodium salt formed as a co-product needs be removed by washing which causes a lot of water waste.

The ion-exchange method is used when M and M′ are not stable in alkaline medium or no suitable soluble salt of the anion AI′ can be found. An LDHs precursor is first synthesized and the ion-exchange reaction is then carried out in the presence of the required interlayer anions under appropriate conditions in order to prepare the target LDHs. In this method, the washing process cannot be omitted due to the formation of salt by-products in the production of the precursor.

In the calcination-rehydration method, complex metal oxides (LDO) are obtained by calcination of an LDHs precursor, and the LDO is added into a solution containing the desired anions to restore or partly restore the ordered layered structure of LDHs. Generally, it is possible to restore the ordered layered structure when the calcination temperature is below 500° C. When the calcination temperature exceeds 600° C., a spinel phase is formed from which the layer structure of the LDHs cannot be restored. An LDHs precursor must also be synthesized for use in the calcination-rehydration method and therefore the washing process cannot be omitted due to the formation of salt by-products.

Certain embodiments of the present invention include layered double hydroxide nanostacks comprising at least one but preferably a plurality of nanosheets represented by the general formula: [M_((1-x))M′_(x)(OH)₂]^(x+), wherein: M is a metal cation in a formal +2 oxidation state; M′ a metal cation in a formal +3 oxidation state; wherein

the nanosheets are associated with or intercalate [A^(m−) _(x/m)], where A is a displaceable anion;

m is an integer (for example 1, 2, 3, or 4);

x is a positive number less than 1;

the nanostack being optionally hydrated with a stoichiometric amount or a non-stoichiometric amount of water; and

the nanosheet having at least one lateral edge dimension in a range of from about 5 nm to about 500 nm.

In other embodiments, the nanostacks may comprise dehydrated mixed metal oxides or hydrated mixed metal oxides, for example, equivalent to sintered layered double hydroxide, and comprise the corresponding mixed metal oxide or hydrated oxide nanocatalysts.

M can be any metal cation, derived from a transition or main group metal, that exists in a formal +2 oxidation state. In separate independent embodiments, M is or comprises at least one of Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Sr²⁺, Zn²⁺, or any combination thereof. In some embodiments, M is independently Cu²⁺, Co²⁺, Fe²⁺, Ni²⁺, Pb²⁺, Sr²⁺, or Zn²⁺. In other embodiments, the mixed oxide or double-hydroxide nanostacks (i.e., excluding the dopants described herein) comprise a single M metal, for example Ni²⁺.

Similarly, M′ can be any metal cation that exists in a formal +3 oxidation state. Non-limiting examples of M′ include at least of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, Y³⁺, Ce³⁺. In separate independent embodiments, M′ is or comprises at least one of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, La³⁺, Mn³⁺, V³⁺, Y³⁺, or Ce³⁺. Again, in some embodiments, the nanostacks (i.e., excluding the dopants described herein) comprise a single M′ metal, for example Fe³⁺. While M and M′ can comprise the same metal in different oxidation states (e.g., Co²⁺/Co³⁺, Fe²⁺/Fe³⁺, or Mn²⁺/Mn³⁺), in most embodiments, the two metals M and M′ are different.

In addition to the template M and M′ cations, in some embodiments, at least one of the nanosheets is further doped with at least one other Lewis acid ion of another transition metal, lanthanide, or actinide metal ion. For example, [Ni—Fe]LDH (where x<0.5, <0.4, or <0.3) may be doped with titanium or lanthanum cations. Typically, such doping is done at a level of 10 atomic %, 5 atomic %, or less, based on the total metal content of the layered double hydroxide.

A may comprise any organic or inorganic anion, or a combination thereof. Again, these anions, at least in the double hydroxide nanostacks, are exchangeable, and that such exchanges are known and documented in the art. Such interchangeability of the intercalations anions may occur either as by deliberate synthesis (i.e., deliberate exchange reactions) or in situ during their use in certain electrochemical environments. Such independent inorganic exemplars for A include F⁻, Cl⁻, Br⁻, I⁻, BF₄ ⁻, PF₆ ⁻, CO₃ ²⁻, HCO₃ ⁻, CrO₄ ²⁻, NO₂ ⁻, NO₃ ⁻, ONO₂ ⁻, ClO₂ ⁻, ClO₃ ⁻, ClO₄.⁻, H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, IO₃ ⁻, OH⁻, S²⁻, SO₃ ²⁻, S₂O₃ ²⁻, SO₄ ²⁻, WO₄ ²⁻, or any combination thereof. Independent inorganic exemplars for A include acetate, propionate, lactate, terephthalate, adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a combination thereof. A may comprise a mixed organic or inorganic anion. A may also comprise a complex anion comprising a transition metal compound (e.g., Mo₇O₂₄ ⁶⁻V₁₀O₂₈ ⁶⁻, PW₁₁CuO₃₉ ⁶⁻, or SiW₉V₃O₄₀ ⁷⁻). When the double hydroxide nanostacks are derived from ablation in the presence of an aqueous solution, A typically comprises hydroxide, as well as the anion associated with the aqueous metal ion (vide infra).

Additionally, these inventive double hydroxide nanostacks may also comprise other neutral or anionic payloads. For example, analogous LDHs have been used as delivery systems for intercalated biocides (fungicides, fungicide, an algicide or a bactericide, as described in U.S. Pat. No. 8,986,445); pharmaceuticals (e.g., 4-biphenylacetic acid, Diclofenac, Gemfibrozil, Ibuprofen, Naproxen, 2-propylpentanoic acid and Tolfenamic acid as described in U.S. Pat. No. 8,709,500 or 8,747,912); pyrithione or a polyvalent metal salt of pyrithione (anti-dandruff; as described in U.S. Pat. No. 8,673,274); or anionic organic or organometallic pigments or colorants as described in U.S. Pat. No. 7,799,126). Such intercalation products are considered within the scope of the present invention.

As described above, x has been defined in terms of a non-zero, positive number less than 1. In other independent embodiments, x is 0.9 or less, 0.8 or less, 0.7 or less, 0.5 or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less. In other embodiments, the layered double hydroxide nanostack may be described in terms of x, wherein x is in a range bounded at the lower end by a value of about 0.1, about 0.05, about 0.1, about 0.15, about 0.2, about 0.25, about 0.3, about 0.35, about 0.4, about 0.45, or about 0.5 and at the upper end by a value of about 0.95, about 0.9, about 0.85, about 0.8, about 0.75, about 0.7, about 0.65, about 0.6, or about 0.5, with non-limiting exemplary ranges including those from about 0.05 to about 0.95, from about 0.20 to about 0.95, from about 0.2 to about 0.35, or from about 0.22 to about 0.95.

The nanostacks have been described as comprising at least one but preferably a plurality of nanosheets, wherein the nanosheet has at least one lateral edge dimension in a range of from about 5 nm to about 500 nm. One of the key features of the inventive laser ablation methods described is the ability to provide such nanodimensioned sheets. In additional embodiments, these nanosheets may have at least one, and preferably two, lateral dimensions ranging from about 5 nm to about 500 nm, 400 nm, 300 nm, 250 nm, 200 nm, 150 nm, 100 nm, 50 nm, or about 25 nm. As shown in the Examples, such nanostacks are conveniently prepared having nanosheets having at least one lateral edge dimension in a range of from about 7 nm to about 22 nm.

Also, the nanostacks have been described in terms of “preferably a plurality of nanosheets.” While such nanostacks includes those embodiments comprising a single nanosheet with its associated anion set, as used herein, the term “plurality” is intended to connote two or more, with specific embodiments providing that this number is at least two, at least three, or at least four.

Independent embodiments include methods of preparing any of the layered double hydroxide nanostacks or related nanoparticle catalysts described herein comprising subjecting a solid ablation target to an energy source, the solid ablation target comprising a first metal capable of oxidizing to a positive oxidation state, the energy source impinging on the first metal in the presence of an aqueous ablation solution containing a second metal ion in a positive oxidation state for a time and energy sufficient to ionize at least a portion of the first metal to a positive oxidation state.

Preferably, but not necessarily, the aqueous ablation solution is substantially free of surfactants. As used herein, the term “substantially free” of surfactants means that there is no added surfactant. This absence of the surfactants within the aqueous ablation solution translates into the absence of surfactants within the layered double-hydroxide nanostacks, which may in part contribute to their superior electrochemical oxidation performance.

These methods are operable whether M is derived the solid ablation target or the second metal metal ion. Likewise M′ may be derived from the solid ablation target or the second metal metal ion. That is, in some embodiments, the first metal is capable of achieving a +2 oxidation state, and the second metal is in a +3 oxidation state. In other embodiments, the first metal is capable of achieving a +3 oxidation state, and the second metal is in a +2 oxidation state. The first metal may comprise beryllium, calcium, cadmium, copper, cobalt, iron, magnesium, manganese, nickel, strontium, zinc, or an alloy or mixture thereof and the second metal ion comprises at least one of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, and Y³⁺. In other embodiments, the first metal comprises aluminum, cerium, cobalt, chromium, iron, gallium, indium, lanthanum, manganese, vanadium, yttrium, or an alloy or mixture thereof, and the second metal comprises at least one of Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺, and Zn²⁺.

In certain specific embodiments, M is Ni and M′ is Fe³⁺.

The energy used for the ablation is conveniently produced by a laser, and in some embodiments a pulsed laser (or a conventional laser adapted to provide the energy in pulses). In certain embodiments, the laser pulse delivers energy at a sufficient flux and to ionize the metal of the laser ablation target. Without intending to be bound by the correctness of any particular theory, it is believed that the energy of the laser causes this transformation by establishing a plasma environment at or near the surface of the laser ablation target, confined by the boundaries imposed by the aqueous environment. In this case, the energy is defined by both the absolute energy and frequency of each pulse and the duration of the treatment. Such conditions are conveniently established when the pulse energy is in a range bounded at the lower end by a value of about 50 mJ/pulse, 70 mJ/pulse, 90 mJ/pulse, 110 mJ/pulse, 130 mJ/pulse, 150 mJ/pulse, 170 mJ/pulse, 190 mJ/pulse, or 210 mJ/pulse and at the upper end by a value of about 500 mJ/pulse, 450 mJ/pulse, 400 mJ/pulse, 350 mJ/pulse, 300 mJ/pulse, 250 mJ/pulse, 230 mJ/pulse, 210 mJ/pulse, or 190 mJ/pulse. Under these conditions, the frequency of the pulse can be provided in a range of from about 10, 20, 30, 40, or 50 Hz for periods of time ranging from minutes to hours. In the Examples provided, pulse energies in a range of from about 90 mJ/pulse to about 210 mJ/pulse were applied at 10 Hz for 60 minutes, corresponding to about 90/210×10⁻³ J×10 sec⁻¹×3600 seconds or about 3.2-7.6 kJ. These parameters are tunable, depending on the nature of materials and the desired quantity and type of materials, and so should not be considered limiting as to the nature of the invention.

Generally, the formed nanostacks incorporate hydroxide (in the case of aqueous ablation media) and the counterions associated with the second metal ion present in the ablation solutions. However, these counterions are displaceable with other counterions, and further processing steps may comprise exchanging these displaceable anions with others by treatments known in the art. Again, these exchanges may be accomplished deliberately (i.e., deliberate exchange reactions) or may be affected in situ during their use in certain electrochemical environments.

Still further embodiments provide that the initially-formed layered double hydroxide nanostack is subjected to further processing. In some of these embodiments, the initially-formed layered double hydroxide nanostack sintered. In certain aspects of this embodiment, the sintering may done in an oxidizing environment, producing mixed oxide or hydrated oxide nanostructures. In other aspects, the sintering is done in an oxidatively inert environment (e.g., under argon or nitrogen), in some cases, again providing mixed oxide or hydrated oxide nanostructures. In still other aspects, the sintering may be done in reducing environment (e.g., under hydrogen), in some cases providing mixed metallic nanostructures.

To this point, the various embodiments or aspects of this invention have been described in terms of the nanostacks and their methods of making, but the invention also comprises those structures which incorporate these nanomaterials. For example, these nanocatalysts or nanostructures may be incorporated into electrochemical devices, for example as coatings used in such devices, and these devices or coatings are considered to be within the scope of the present invention. For example, certain embodiments of the invention provide for the incorporation of these nanomaterials into templates, polymers, or coatings. For example, these embodiments may include electrodes, each comprising a coating comprising one or more of one or more of the inventive layered double hydroxide nanostacks, or mixed oxide or hydrated oxide nanostacks. When the layered double hydroxide nanostack are incorporated into a coating of an electrode, especially the [NiFe]LDH structures described herein, that electrode shows excellent properties as an electrochemicals water oxidation catalyst.

Such coatings may be applied to electrode comprises gold, nickel, platinum, or an allotrope of carbon. In some embodiments, the electrodes comprise graphite, graphene, glassy (or vitreous) carbon, diamond, or a combination thereof.

In certain of these embodiments, the coated electrodes show superior performance as water oxidation catalysts, for example being capable of exhibiting or actually exhibiting an overpotential for the oxidation of water to oxygen of less than 300 mV at 10 mA/cm² in use. In certain embodiments, these coated electrodes provide overpotentials of within 290, 280, and 270 mV, perhaps to as low as 250 or 200 mV, of the thermodynamically determined potential for the oxidation of water to oxygen, at 10 mA/cm². It is not clear that the lower boundary of these overpotentials have been realized. These coated electrodes operable and can achieve these low overpotentials across a broad pH range (e.g., from 0 to about 2, from about 2 to about 4, from about 4 to about 6, from about 6 to about 8, from about 8 to about 10, from about 10 to about 12, from about 12 to about 14, or any combination of these ranges), but given the nature of chemical nature of the LDHs, and their ability to retain their crystallinity, longer lasting performance is expected in alkaline conditions; i.e., at pHs ranging from about 7, 7.5, 8, 8.5, 9, 9.5, or 10 to about 14, 13.5, 13, 12.5, 12, 11.5, 11, 10.5, or 10. Again, the longevity of these electrodes will depend on the specific nature of the chosen LDHs and the operating conditions of the electrochemical reactions (e.g., pH and temperature).

Additional embodiments include the electrochemical cell or cells comprising these inventive electrode, as well as power systems incorporating the electrochemical cell or cells.

Also within the scope of the present invention are those methods for operating these electrodes or electrochemical cells in electrocatalysis. For example, certain exemplary embodiments include those methods for oxidizing water comprising applying a potential to any of the inventive electrodes and passing sufficient current to oxidize water to form oxygen. The person of ordinary skill in the art would appreciate how to operate such devices and methods without undue experimentation. These methods include operating the cell or device such that the potential is within 300 mV (or any of the ranges cited above) of the thermodynamically determined potential for the oxidation of water to oxygen at 10 mA/cm². It should be recognized that reference to 10 mA/cm² is intended only to refer one basis for claims to the superior overpotential performance, and that any reasonable current density may be used in conjunction with these cells and devices.

The following listing of embodiments is intended to complement, rather than displace or supersede, the previous descriptions.

Embodiment 1

A layered double hydroxide nanostack comprising a plurality of nanosheets represented by the general formula: [M_((1-x))M′_(x)(OH)₂]^(x+), wherein: M is a metal cation in a formal +2 oxidation state; M′ a metal cation in a formal +3 oxidation state; wherein

the nanosheets are associated with or intercalate [A^(m−) _(x/m)], where A is a displaceable anion;

m is an integer; [e.g., 1, 2, 3, or 4]

x is a positive number less than 1; [preferably, but not necessarily <0.5]

the nanostack being optionally hydrated with a stoichiometric amount or a non-stoichiometric amount of water; and

the nanosheet having at least one lateral edge dimension in a range of from about 5 nm to about 100 nm.

Embodiment 2

The layered double hydroxide nanostack of Embodiment 1, wherein M is at least one of Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺, and Zn²⁺, and M′ is at least of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, Y³⁺, Ce³⁺

Embodiment 3

The layered double hydroxide nanostack of Embodiment 1 or 2, wherein A comprises an organic or inorganic anion, or a combination thereof.

Embodiment 4

The layered double hydroxide nanostack of Embodiment 3, wherein A comprises F⁻, Cl⁻, Br⁻, I⁻, BF₄ ⁻, PF₆ ⁻, CO₃ ²⁻, HCO₃ ⁻, CrO₄ ²⁻, NO₂ ⁻, NO₃ ⁻, ONO₂ ⁻, ClO₂ ⁻, ClO₃ ⁻, ClO₄.⁻, H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, IO₃ ⁻, OH⁻, S²⁻, SO₃ ²⁻, S₂O₃ ²⁻, SO₄ ²⁻, WO₄ ²⁻, or a combination thereof.

Embodiment 5

The layered double hydroxide nanostack of Embodiment 3, wherein A comprises acetate, propionate, lactate, terephthalate, adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a combination thereof.

Embodiment 6

The layered double hydroxide nanostack of Embodiment 3, wherein A comprises a complex anion comprising a transition metal compound.

Embodiment 7

The layered double hydroxide nanostack of any one of Embodiments 1 to 6, wherein M is or comprises Ni²⁺.

Embodiment 8

The layered double hydroxide nanostack of any one of Embodiments 1 to 7, wherein M′ is or comprises Fe³⁺.

Embodiment 9

The layered double hydroxide nanostack of any one of Embodiments 1 to 8, wherein x is in a range of from 0.05 to 0.95.

Embodiment 10

The layered double hydroxide nanostack of any one of Embodiments 1 to 9, wherein A is or comprises hydroxide.

Embodiment 11

The layered double hydroxide nanostack of any one of Embodiments 1 to 10, wherein the nanosheet has at least one lateral edge dimension in a range of of from about 5 nm to about 25 nm.

Embodiment 12

The layered double hydroxide nanostack of any one of Embodiments 1 to 11, wherein at least one of the nanosheets is further doped with at least one Lewis acid ion of a transition metal, lanthanide, or actinide metal ion.

Embodiment 13

The layered double hydroxide nanostack of any one of Embodiments 1 to 12, wherein at least one of the nanosheets is further doped with titanium or lanthanum.

Embodiment 14

A method (of preparing a layered double hydroxide nanostack of any one of Embodiments 1 to 13) comprising subjecting a solid ablation target to an energy source, the solid ablation target comprising a first metal capable of oxidizing to a positive oxidation state, the energy source impinging on the first metal in the presence of an aqueous ablation solution containing a second metal ion in a positive oxidation state for a time and energy sufficient to ionize at least a portion of the first metal to a positive oxidation state.

Embodiment 15

The method of Embodiment 14, wherein the aqueous ablation solution is substantially free of surfactants.

Embodiment 16

The method of Embodiment 14, wherein the first metal is capable of achieving a +2 oxidation state, and the second metal is in a +3 oxidation state.

Embodiment 17

The method of Embodiment 14, wherein the first metal is capable of achieving a +3 oxidation state, and the second metal is in a +2 oxidation state.

Embodiment 18

The method of Embodiment 16, wherein the first metal comprises beryllium, calcium, cadmium, copper, cobalt, iron, magnesium, manganese, nickel, lead, strontium, zinc, or an alloy or mixture thereof and the second metal ion comprises at least one of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, or Y³⁺.

Embodiment 19

The method of Embodiment 17, wherein the first metal comprises aluminum, cerium, cobalt, chromium, iron, gallium, indium, lanthanum, manganese, vanadium, yttrium, or an alloy or mixture thereof, and the second metal comprises at least one of Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺, or Zn²⁺. In certain of aspects of this Embodiment, M is Fe and M′ is Ni²⁺.

Embodiment 20

The method of any one of Embodiments 14 to 19, wherein the energy is a laser.

Embodiment 21

The method of Embodiment 20, wherein the laser energy is provided in pulses

Embodiment 22

The method of Embodiment 21, wherein the laser pulse delivers an energy in a range of from about 90 mJ/pulse to about 210 mJ/pulse.

Embodiment 23

The method of any one of Embodiments 14 to 22, wherein the displaceable ion A of the layered double hydroxide nanostack comprises a counterion associated with the second metal ion in the aqueous ablation solution.

Embodiment 24

The method of Embodiment 23, further comprising exchanging the displaceable ion A of the layered double hydroxide nanostack with a different anion.

Embodiment 25

The method of any one of Embodiments 14 to 24, wherein the initially-formed layered double hydroxide nanostack is further sintered. In certain aspects of this Embodiment, the sintering is done in an oxidizing environment. In other aspects of this Embodiment, the sintering is done in an oxidatively inert environment. In still other aspects of this Embodiment, the sintering is done in reducing environment.

Embodiment 26

A composition prepared by any one of Embodiments 14 to 25.

Embodiment 27

An electrode comprising a coating comprising the layered double hydroxide nanostack of any one of Embodiments 1 to 13.

Embodiment 28

The electrode of Embodiment 27, wherein the electrode comprises gold, nickel, platinum, or an allotrope of carbon.

Embodiment 29

The electrode of Embodiment 27, wherein the electrode comprises graphite, graphene, glassy (or vitreous) carbon, diamond, or a combination thereof.

Embodiment 30

The electrode of Embodiment 27 or 29, that exhibits an overpotential for the oxidation of water to oxygen of less than 300 mV at 10 mA/cm² on a flat supporting electrode.

Embodiment 31

An electrochemical cell comprising an electrode of any one of Embodiments 27 to 30.

Embodiment 32

A method for oxidizing water comprising applying a potential to an electrode of any one of Embodiments 27 to 30 and passing sufficient current to oxidize water to form oxygen.

Embodiment 33

The method of Embodiment 32, wherein the potential is within 300 mV of the thermodynamically determined potential for the oxidation of water to oxygen at 10 mA/cm².

EXAMPLES

The following Examples are provided to illustrate some of the concepts described within this disclosure. While each Example is considered to provide specific individual embodiments of composition, methods of preparation and use, none of the Examples should be considered to limit the more general embodiments described herein.

In the following examples, efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, etc.) but some experimental error and deviation should be accounted for. Unless indicated otherwise, temperature is in degrees C., pressure is at or near atmospheric.

Example 1 Overview

In certain exemplary, non-limiting, embodiments, surfactant-free mixed-metal hydroxide water oxidation nanocatalysts can be synthesized by pulsed-laser ablation in liquids. In a series of [Ni—Fe]-layered double hydroxides with intercalated nitrate and water, [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O, higher activity was observed as the amount of Fe decreased to 22%. Addition of Ti⁴⁺ and La³⁺ ions further enhanced electrocatalysis, with a lowest overpotential of 260 mV at 10 mA cm⁻². Electrocatalytic water oxidation activity increased with the relative proportion of a 405.1 eV N 1s (XPS binding energy) species in the nanosheets.

Specific embodiments include surfactant-free, highly active [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O nanosheet water oxidation catalysts with admixed ions. The best catalyst had an overpotential of 260 mV at 10 mA cm⁻² on flat highly-ordered pyrolytic graphite working electrodes. The higher activity may be attributable to unique morphological and structural properties, which were synthetically accessible by the use of pulsed-laser ablation in liquids (PLAL). In PLAL, nanoparticles are formed by very rapid cooling of a plasma comprised of elements from the solid ablation target and the surrounding liquid. This condensation process, which is kinetically controlled, produces predominantly crystalline nanomaterials. PLAL offers size and composition control through a wide range of tunable experimental parameters.

With PLAL, mixed-metal nanomaterials with tailored compositions can be prepared readily by adding metal ions into the aqueous ablation liquid. Different amounts of Fe were intentionally incorporated into α-Ni(OH)₂ nanocatalysts, as variable concentrations of Fe in electrodeposited nickel (oxy)hydroxides have been shown to improve electrocatalytic activity. Ti⁴⁺ and La³⁺ ions were also added to the ablation liquid and screened the resulting materials for water oxidation activity.

Eight mixed-metal catalysts were synthesized using PLAL by varying ablation targets, metal ion type and concentrations, and laser pulse energies (see SI for experimental details, all ablation solutions contained nitrate). The nanomaterials were prepared with Fe concentrations ranging from 22 to 95% of the total metal content (Table 1). We identified their compositions spectroscopically; and, notably, they all exhibited high electrocatalytic oxygen-evolution activities in basic electrolytes.

Powder X-ray diffraction (XRD) measurements indicated that the Fe-rich nanoparticles 1-3 were poorly crystalline; the Ni-rich nanoparticles 4-8 displayed diffraction patterns consistent with layered double hydroxide (LDH) structures. XRD data indicated minor contributions from Fe(O)OH; 6 also contained the crystalline spinel NiFe₂O₄, and Ti-based oxides were present in 7 and 8. LDHs have the general formula [M_(1-x)M′_(x)(OH)₂](A^(m-))_(x/m).nH₂O; the structures are comprised of sheets of [M_(1-x)M′_(x)(OH)₂]^(x+) edge-shared octahedra. Cationic charges arising from M′³⁺ in the sheets are balanced by intercalated hydrated anions (A^(m−)).

TABLE 1 Preparation conditions of catalysts 1 to 8 and concentrations of Fe with respect to total metal content. Ion con- Pulse Fe Solid Added centration energy (% metal Catalyst target ions (M) (mJ) content)^(a) 1 Ni Fe 0.1 90 95 2 Ni Fe 0.01 90 86 3 Fe Ni 0.1 90 70 4 Fe Ni 1.0 90 36 5 Fe Ni 3.0 90 22 6 Fe Ni 3.0 210 30 7 Fe Ni 3.0 210 23 Ti 0.015 8 Fe Ni 3.0 210 29 Ti 0.015 La 0.023 ^(a)Determined by XPS

X-ray photoelectron spectroscopy (XPS) was employed to obtain binding energies of Ni 2p and Fe 2p core levels in 1-8; these energies were indicative of Ni(OH)₂ and (hydrous) iron oxides. In addition, Mössbauer and x-ray absorption spectroscopic data indicated that Fe was incorporated as Fe³⁺ in place of Ni²⁺ in [Ni—Fe]-LDHs. Two well-resolved N 1s peaks appeared in the XP spectra of nanoparticles 4-8, with binding energies of 407.3 and 405.1 eV. The higher binding-energy feature (407.3 eV) was assigned to nitrate. The 2.2 eV reduction in N 1s binding energy for the second feature could have arisen from nitrate in an unusual electronic environment, although nitrogen in a lower oxidation state (e.g., NO₂, NO₂ ⁻) could not be ruled out. Infrared spectra were consistent with the presence of a second type of NO_(x) species. Infrared and Raman data supported the presence of intercalated nitrate anions in the LDH structure. On the basis of these data, the predominant crystalline material in 4-8 was assigned to the [Ni— Fe]-LDH [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O (FIG. 1).

Nanoparticle sizes were obtained from transmission electron micrographs (TEM), and crystalline domain sizes were determined by Scherrer analysis of XRD data. Lateral sizes ranged from ˜7 to 22 nm (Table 2). Catalysts 1 to 5 consisted of nanosheets, as expected for layered structures. Analysis of TEM and XRD data for 6 revealed that two types of nanoparticles were formed; smaller, more spherical (6.5±0.8) nm particles are attributed to the spinel NiFe₂O₄, and larger (13±1) nm sheets are assigned to the LDH [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O. Also, differences in TEM contrast, shape, and size were found for 7 and 8. Specific surface areas of catalysts 5 to 8 determined by Brunauer-Emmett-Teller (BET) measurements are in agreement with particle sizes derived from TEM data. Catalysts 6 to 8, which were synthesized at 210 mJ pulse energy, had similar BET surface areas (193±1 m² g⁻¹), whereas 5, prepared at 90 mJ/pulse, exhibited a slightly higher surface area (220 m² g⁻¹).

The electrocatalytic oxygen-evolution activity was assessed in 1 M aqueous KOH. Faradaic yields of oxygen evolution for 5, 6 and 8 were all essentially 100%. Steady-state Tafel data were measured to obtain overpotentials; virtually identical mass loadings were used in all electrochemical experiments (all current densities are reported per geometric area). Importantly, chronoamperometry data showed that the catalytic activity of nanoparticles 5-8 was maintained for more than 5 hours.

The electrocatalytic activities of materials 1 to 5, synthesized at virtually the same pulse energy, steadily increased with decreasing Fe content (FIG. 2). Catalyst 5 (22% Fe relative to total metal content) performed best in the [Ni—Fe]-LDH materials, with an overpotential of 280 mV at 10 mA cm⁻². Incorporation of less than 22% Fe relative to total metal content was limited by the solubility of Ni nitrate in the aqueous ablation liquid. XRD data for 5, collected before and after 30 min of anodic polarization, confirmed that the crystallinity of the [Ni—Fe]-LDH material was retained (FIG. 3). The Fe content of our best performing catalyst is in agreement with previous reports. It differed, however, from findings for amorphous materials, which performed best with 40% Fe.

Catalyst 6 was made employing virtually the same precursor conditions as for 5, but with a pulse energy of 210 instead of 90 mJ. It has been shown previously with cobalt oxide that pulse energy can be used to control particle size. Varying pulse energy in the synthesis of more complex mixed-metal materials led to particles with different compositions (FIG. 4A-C). While 5 consisted mainly of crystalline [Ni—Fe]-LDH, 6 was mixed crystalline [Ni—Fe]-LDH/NiFe₂O₄. Catalyst 6 showed inferior activity for water oxidation relative to 5, presumably because the active [Ni—Fe]-LDH was diluted by the spinel oxide. This finding suggested that crystalline [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O is the more active species in our materials for catalytic water oxidation. IR spectra of 5 and 6 were consistent with [Ni_(1-x)Fe_(x)(OH)₂](NO₃)_(y)(OH)_(x-y).nH₂O with high interstitial water and nitrate content. The positions of peaks in the IR spectrum of catalyst 5 indicated the incorporation of Fe into the α-Ni(OH)₂ lattice.

The addition of Lewis-acidic Ti⁴⁺ and La³⁺ ions to the ablation liquid was found to improve catalytic activity relative to the most active [Fe—Ni]-LDH catalyst (5). Catalysts 7 and 8 were synthesized using virtually the same precursor conditions as for 5, but with Ti⁺ (7) or Ti⁴⁺ and La³⁺ (8) added to the ablation solution (Table 1). XRD data revealed that both catalysts were primarily [Ni—Fe]-LDH materials. Oxides containing added elements were also present; TiO₂ and Fe₂TiO₄ were found in 7, whereas crystalline Ni₃TiO₅ and La(Ni,Fe)O₃ were detected in 8. The spinel oxide NiFe₂O₄ was absent from both 7 and 8. XPS data showed that 8 contained 1% La relative to total metal content. Both catalysts were more active than LDHs 5 and 6, with 7 and 8 exhibiting the lowest overpotentials at 10 mA cm⁻² of 270 and 260 mV, respectively.

Highly active, surfactant-free, mixed transition metal hydroxide water oxidation nanoparticle catalysts can be made by PLAL. Acrystalline [Ni—Fe]-LDH was spectroscopically identified as the catalytically most active material. The turnover frequency (TOF) was found to correlate with the ratio of two nitrogen species detected by XPS in the as-synthesized catalysts (FIG. 5A-B). Addition of Ti⁴⁺ and La³⁺ ions further enhanced activity (reaching 10 mA cm⁻² at an overpotential of 260 mV). On a flat electrode, this is the lowest overpotential reported to date for mixed metal oxide catalysts.

Example 2 General Experimental Conditions and Apparatus Example 2.1 Materials and Methods

Nanomaterial synthesis by pulsed laser ablation in liquids was performed in the Beckman Institute Laser Resource Center at California Institute of Technology. X-ray photoelectron spectroscopy was carried out at the Molecular Materials Research Center (Beckman Institute at California Institute of Technology). Transmission electron micrographs were collected at the Beckman Resource Center for Transmission Electron Microscopy (California Institute of Technology).

All chemicals were used as received. Deionized water was obtained from a Barnstead Diamond Nanopure system and had a resistivity of ≧16 MΩ cm⁻¹.

Example 2.2 Synthesis

Mixed metal nanomaterials were synthesized using the method of pulsed laser ablation in liquids (PLAL). Suspensions of iron (Alfa, −200 mesh, 99+%) or nickel (Alfa, −150+200 mesh, 99.8%) powders were stirred in 10 mL aqueous metal nitrate solutions using a magnetic stirrer in a 30 mL glass beaker at room temperature in ambient air. For ablation, 0.5 g iron powder or 2.0 g nickel powder were used. With iron as ablation target, the liquid consisted of 10 mL pH 10.0 water (adjusted with potassium hydroxide, Mallinckrodt) with nickel nitrate (Ni(NO₃)₂.6H₂O, Alfa, 98%) concentrations of 0.1 M, 1.0 M, and 3.0 M. With nickel as ablation target, the liquid was 10 mL water with iron nitrate (Fe(NO₃)₃.9H₂O, Alfa, 98.0-101.0%) concentrations of 0.01 M and 0.1 M. Nanomaterials with more than two metals were made from 0.5 g iron powder suspended in 10 mL of a solution of 3.0 M nickel nitrate and 0.015 M titanium(IV) oxide bis(acetylacetonate) (Strem, >95%) in 10 mL pH 10.0 water (adjusted with KOH). Some solutions also contained 0.023 M lanthanum nitrate (La(NO₃)₃.6H₂O, Sigma-Aldrich, ≧99%). Beakers and stir bars were thoroughly cleaned with aqua regia before use.

A 355 nm, 8 ns pulse laser beam, provided by the third harmonic of a 10 Hz Q-switched Nd:YAG laser (Spectra-Physics Quanta-Ray PRO-Series), was focused 0.5 mm below the surface of the liquid with a 100 mm focal length plano-convex quartz lens. Each sample was irradiated for 60 min. Laser pulse energies were either 90 or 210 mJ/pulse.

After synthesis, nanoparticle suspensions were separated from the metallic ablation targets using a strong magnet. Solid nanoparticulate powders were obtained by centrifugation and washing with water until the supernatant did not show any metal nitrate absorption. The nanoparticles were then washed twice with acetone (EMD, OmniSolv®) and dried under vacuum. A high precision balance (Sartorius CPA225D) was used to weigh the nanoparticle powders. Around 5 mg material were suspended in water to make 2 mg mL-1 suspensions; 20 μL of these were drop cast on freshly-cleaved highly-ordered pyrolytic graphite (HOPG) working electrodes and dried in ambient air under a heat lamp at 50° C., resulting in a catalyst loading of 40 μg.

Electrodeposited nickel oxide catalyst was prepared according to the procedure published by Dinc{hacek over (a)}, M. et al., Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 10337. In detail, 2.18 g Ni(NO₃)₂.6H₂O was dissolved in 5 mL water and added to 75 mL rapidly stirred 0.1 M pH 9.20 aqueous sodium borate buffer, which immediately became turbid. The sodium borate buffer was made from sodium tetraborate (Na₂B₄O₇.10H₂O, Baker, 101.4%) and its pH was adjusted by adding boric acid (H₃BO₃, Mallinckrodt, 99.9%). The filtrate of the suspension was used as the electrolyte; the working electrode was a freshly cleaved HOPG electrode. An Ag/AgCl/3.0 M NaCl reference electrode (Bioanalytical Systems, Inc.; measured to be +0.212 V vs NHE, and a Ni gauze (Alfa) counter electrode were used. A 51 mC charge was passed with an applied voltage of 1.312 V vs. NHE; faradaically, we deposited 530 nmol Ni, which corresponds to 40 μg NiO. Before catalytic activity testing the electrodeposited films were thoroughly washed with water.

Example 2.3 Physical Characterization

X-ray photoelectron spectra (XPS) were taken using a Surface Science Instruments M-probe surface spectrometer. Monochromatic Al Kα radiation (1486.6 eV) was used to excite electrons from the samples, which had been drop-cast on clean Cu foil and dried in ambient air at room temperature. The sample chamber was maintained at <5×10⁻⁹ Torr. Survey scans from 0 to 1000 eV were carried out to identify the elements present in the nanoparticles. Binding energies were referenced to the C 1s peak arising from adventitious carbon, taken to have a binding energy of 284.8 eV. See Barr, T. L. et al., J. Vac. Sci. Technol. A 1995, 13, 1239. High-resolution spectra were collected for the Fe 2p, Ni 2p, Ti 2p, La 3d, N 1s, and O 1s regions. Quantitative peak areas were derived after Shirley background subtraction (see Shirley, D. A. Phys. Rev. B 1972, 5, 4709) and using relative sensitivity factors. Binding energies were obtained from the same peak fits. Quantitative XPS analysis was performed with CasaXPS (Version 2.3.16 PR 1.6).

X-ray diffraction (XRD) data were collected with a Bruker D2 PHASER diffractometer. Monochromatic Cu Kα radiation (1.5418 Å; tube power 30 kV, 10 mA) was used; the instrument was equipped with 0.1° divergence, 1.5° Soller, and 0.6 mm detector slits, and had a 3-mm secondary anti-scatter screen. Diffracted radiation was collected with a Lynxeye detector. The instrument resolution was 0.05° in 2θ, and the counting time was 3 seconds per step, resulting in a total scan time of about 75 min for each sample. Solid samples were deposited with vaseline (X-Alliance GmbH) on a zero-diffraction silicon plate (MTI Corporation). XRD background subtraction, Scherrer and pattern analysis were performed with the Bruker DIFFRAC.SUITE software coupled to the International Centre for Diffraction Data powder diffraction file database (ICDD, PDF-2 Release 2012).

Raman spectra of neat solid catalysts were collected at room temperature in ambient air with a Renishaw M1000 micro-Raman spectrometer. A 50× magnification objective and a 50-μm slit, resulting in 4 cm-1 resolution, were used. The laser excitation wavelength was 514.3 nm (Cobolt Fandango™ 100 laser), the power at the sample was 213 μW (1% laser power, measured with a Thorlabs PM100USB power meter), and depolarized scattered light was detected. The excitation intensity was chosen as to prevent radiation damage of the nanoparticulate powders; collected spectra did not change during repeated scans. The radiation damage threshold was approximated to be at a laser intensity that was three times higher than that applied. Application of 10% laser power through a 50× magnification objective led to immediate radiation damage, and a dark spot was visible on the sample when viewed through the microscope. Focusing the 10% power laser beam through a 20× magnification objective led to gradual sample degradation over multiple scans, which was also observed by visual inspection with the microscope. The instrument's autofocus function was used to maximize the signal-to-noise ratio. The accumulation time was 10 s, and 8 scans were averaged for each sample. The measured Raman shifts were calibrated against a Si standard. Spectra were compared to reference spectra from the RRUFF database (Downs, R. T. The RRUFF Project: an integrated study of the chemistry, crystallography, Raman and infrared spectroscopy of minerals. Program and Abstracts of the 19th General Meeting of the International Mineralogical Association in Kobe, Japan. O03-13, 2006), which were collected with 532 nm excitation and depolarized detection.

Attenuated total reflectance infrared spectra of neat nanoparticulate powders were collected with a Thermo Nicolet iS50 FT-IR spectrometer, equipped with a Pike Technologies GladiATR accessory plate and an uncooled pyroelectric deuterated triglycine sulfate (DTGS) detector. In the 50 to 700 cm-1 range, a far-infrared multilayer beamsplitter was used and a measured water vapor spectrum was subtracted from the data; in the 400 to 4000 cm-1 range, a KBr beamsplitter was used. Spectra of the solid nanoparticulate powders were collected at room temperature in ambient air, and 132 scans were averaged for each sample. Transmission electron microscopy (TEM) measurements were performed with an FEI Tecnai T-12. For each material, 2 μL of a suspension of 2 mg mL-1 nanoparticles in water were drop cast on a 200 mesh Cu grid coated with Formvar carbon (Ted Pella), which was placed on a Kimwipe. The nanoparticles were dispersed on the hydrophobic grid surface by adding 10 μL isopropanol. The average diameter of the nanoparticles was determined using the ImageJ software.

Specific surface areas were determined by Brunauer-Emmett-Teller (BET) measurements, using a Quantachrome Autosorb iQ instrument. Adventitious adsorbates were removed under vacuum by heating approximately 40 mg of each catalyst powder at a rate of 10 K min-1 from room temperature to 423 K, holding it there for 1 hour, followed by heating to 573 K at a rate of 10 K min-1, where it remained for 6 hours, and subsequent cooling to room temperature. Multipoint argon adsorption-desorption isotherms were collected at 87.45 K, and the specific surface areas were calculated with the instrument's built-in software, based on the BET equation.

Example 2.4 Electrochemical Characterization

Cyclic voltammetry, Tafel, and chronoamperometry data were collected at room temperature. For all electrochemical measurements, the electrolyte was aqueous 1.0 M pH 14.0 KOH (Mallinckrodt); an Hg/HgO reference electrode (CH Instruments), a Ni gauze (Alfa) counter electrode, and HOPG working electrodes with 40 μg catalyst on them were used. Working electrodes for cyclic voltammetry, faradaic oxygen yield, and chronoamperometry data consisted of upward-facing HOPG (GraphiteStore, surface area: 0.09 cm²) electrodes. Their preparation is described in Blakemore, J. D. et al., ACS Catal. 2013, 3, 2497; the only difference was that the glass tubes were u-shaped at one end to make the HOPG electrode surface face upwards, which facilitated measurements with extensive oxygen evolution because it allowed the generated gas to bubble up. Working electrodes were cleaned by sonication for 10 min in concentrated hydrochloric acid, washed with water, and their surfaces were polished using 400 and 600 grit sandpaper, after which the graphite was cleaved with adhesive tape to obtain a fresh HOPG surface for each catalyst.

Cyclic voltammograms were measured at 10 mV s⁻¹ scan rate with a Gamry Reference 600 potentiostat. Tafel data were recorded using a rotating disk electrode (RDE) apparatus. Measurements were carried out in a 100 mL three-neck round-bottom flask with a Pine MSR variable speed rotator used at 1,500 rpm and a Princeton Applied Research Parstat 4000 potentiostat. The dwell time at each applied potential point was 5 min to reach steady-state conditions. The disk electrode was made of HOPG with stabilizing epoxy around its side (surface area: 0.13 cm²). The current density versus potential data were post-measurement corrected for uncompensated resistance losses (see below). All potentials reported here are relative to the normal hydrogen electrode (NHE), and current densities are per geometric area.

The ohmic drop (uncompensated resistance, Ru) was experimentally determined for an HOPG working electrode, either blank or with 40 μg nanoparticulate catalyst loading, using a Gamry Reference 600 potentiostat and its built-in “measure Ru” utility that uses the current interrupt method. The working electrode was swept between 0.107 and 0.907 V vs. NHE and Ru values were collected. The averages of 3 Ru values were plotted as a function of applied potential, and the data were fit with a line (FIG. 6).

Post-measurement iR drop correction was performed according to Oelβner, W., et al., Mater. Corros. 2006, 57, 455. This method was chosen, where R_(u) for the nanoparticulate catalysts was experimentally determined under the same conditions as all other electrochemical measurements, because automatic iRcorrection is inherently problematic for high-surface-area materials. In detail, the true polarization potential E_(p) was calculated from the applied potential E_(a), the measured current i, and the uncompensated resistance R_(u) as E_(p)=E_(a)−iR_(u).

Faradaic yields of oxygen evolution data were collected with an apparatus described in Blakemore, J. D. et al., ACS Catal. 2013, 3, 2497. A glass cell was filled with 65 mL electrolyte, leaving 59 mL headspace, in which the O₂ concentration was measured. A potential of 0.857 V vs. NHE was applied for 30 min, using a Gamry 600 potentiostat. The electrolysis chamber was water-jacketed and kept at a constant temperature of (22.0±0.5°) C. to ensure a stable response from the O₂ sensor. In a typical experiment, based on the charge transferred, we expected 284 μL of O2 evolved and detected 297 μL. This confirmed essentially 100% oxygen evolution within the error (10%) of our method.

Long-term stability measurements were performed using a Gamry 600 potentiostat and a working electrode, onto which 40 μg catalyst had been drop cast from a 2 mg mL⁻¹ suspension that also contained 80 μg mL⁻¹ Nafion 117 (Aldrich). Nafion was added for chronoamperometry experiments to improve the mechanical stability of catalyst films on HOPG during oxygen evolution. A voltage of 0.654 V vs. NHE was applied for 5.5 hours and the current was recorded. Data analysis and graphing was performed with Igor Pro 6.34 (Wavemetrics).

Example 3 Physical Characterization Example 3.1 X-Ray Photoelectron Spectra

XPS data were collected to identify nanoparticle compositions by peak integrations of high-resolution spectra of the Fe 2p, Ni 2p, O 1s, N 1s, Ti 2p, and La 3d regions, where applicable. The regions were chosen as to collect data on transitions with the highest x-ray ionization cross-sections. Since the x-ray ionization cross-section of Ti 2p is a factor of 5.4 lower that that of La 3d, and 1.5 times less Ti⁴⁺ than La³⁺ was added to the ablation liquid, no Ti photoelectrons were detected. We deliberately did not attempt to quantify oxygen content from XPS data because the amount of this element is regularly overestimated; oxygen also occurs in other sources, such as adventitious carbon species and oxides of the underlying copper substrate. See FIG. 7 and FIG. 8.

The Fe 2p core level spectra of catalysts 1 to 8 showed peaks consistent with iron oxides and oxyhydroxides, with Fe 2p_(3/2) binding energies close to 710.9 eV. It is not possible to distinguish the different Fe phases in our materials from Fe 2p XPS data, as various iron oxides and oxyhydroxides, such as FeO, Fe₂O₃, Fe₃O₄, and FeOOH, have similar Fe core-level binding energies and spectral shapes. The Ni 2p core-level binding energies of catalysts 1 to 8 were indicative of Ni(OH)₂ or NiOOH, with Ni 2p_(3/2) binding energies close to 855.7 eV. The O 1s spectra of 1 to 8 exhibited, among contributions from adventitious oxygen species, two peaks centered around 528.8 eV and 531.4 eV, as expected for Fe or Ni oxide and hydroxide species, respectively. 10 The N 1s core level spectra of catalysts 1 to 8 showed peaks with binding energies above 405 eV, ascribable to nitrate. The N 1s peaks at 407.3 eV were assigned to surface-adsorbed nitrate, in accord with a report on nitrate adsorbed on hematite, and the peaks centered at 405.1 eV were attributed to interstitial nitrate.

Example 3.2 X-Ray Diffraction Data

XRD data were collected to determine crystalline phases and crystallite sizes by Scherrer analysis. Note that peak widths were determined by factoring in multiple diffraction lines from the corresponding PDF, where applicable. Overlapping diffraction lines may give rise to peaks that appear broader in the total intensity spectra. As a result, peak broadness in the total intensity spectrum does not necessarily correlate to the actual crystalline domain size. Crystalline phases were assigned using the automatic search/match function of the Bruker software DIFFRAC.SUITE. The Fe-rich catalysts were amorphous, 1 and 3 completely so, and 2 predominantly so, with some broad peaks that were assigned to poorly crystallized magnetite and maghemite. XRD data of the more Ni-rich catalysts 4 to 8 showed mainly crystalline α-Ni(OH)₂ (jamborite) and a minor contribution from crystalline FeOOH (goethite). We could not observe any β-Ni(OH)₂ (theophrastite) in our catalysts. Catalyst 6 additionally contained crystalline NiFe₂O₄ (trevorite). In 7 and 8, minerals containing added elements were also present; TiO₂ and ulvospinel (Fe₂TiO₄) were detected in 7, while crystalline Ni₃TiO₅ and La(Ni,Fe)O₃ were found in 8. The characterization of these crystalline material phases were made by comparison with literature data. See FIG. 9 and FIG. 3.

Example 3.3 Transmission Electron Micrographs

TEM images were taken to obtain nanoparticle sizes. FIG. 10. The intention was to avoid blocking catalytically active surface sites; therefore the nanoparticles were synthesized by PLAL without any surfactants. They naturally aggregated in aqueous suspensions. Very dilute samples were prepared on TEM grids, resulting in only a few (aggregated) nanoparticles being imaged per frame. Note that frame-filling nanoparticle patterns will only form by self-assembly of surfactant-capped nanoparticles due to repulsive or attractive forces between surfactant molecules.

Nanocatalyst compositions and sizes are summarized in Table 2. Compositions were derived from XPS peak area quantification. Scherrer analysis of XRD data for catalysts 4 to 8 was used to obtain crystalline domain sizes (materials 1 to 3 were poorly crystallized); the corresponding crystalline phases are given in parentheses. Nanoparticle sizes were determined by TEM image analysis.

Analysis of TEM and XRD data of 6 suggested that smaller, (6.5±0.8) nm particles could be attributed to trevorite, and larger (13±1) nm nanosheets could be assigned to jamborite. It became evident from inspection of TEM images of 6 that the smaller (trevorite) nanoparticles exhibited more contrast, consistent with more spherical shape, than the larger jamborite sheets. Trevorite is a spinel that crystallizes in the cubic system, rendering the formation of nanoparticles with radial symmetry likely. Jamborite, however, crystallizes as a layered structure, leading to axially elongated nanosheets. Likewise, differences in TEM contrast, shape, and size were found for catalysts 7 and 8.

TABLE 2 Catalyst metal contents, concentrations of interstitial and surface-adsorbed nitrate with respect to total metal content, crystalline domain sizes, and nanoparticle sizes. % % Nitrogen Nitrogen (405.1 (407.3 Nano- eV eV Crystalline particle % Metal binding binding Domain Size Size Catalyst Fe Ni La energy) energy) (nm) (nm) 1 95  5 — 0 0 — 22 ± 3  2 86 14 — 0 8 — 10 ± 2  3 70 30 — 1 6 — 7.7 ± 2   4 36 64 — 6 10  12 ± 3 (LDH) 14 ± 2  5 22 78 — 5 5 9 ± 2 (LDH) 12 ± 2  6 30 70 — 5 5 13 ± 3 (LDH) 13 ± 2  6.1 ± 0.5 (spinel) 6.5 ± 0.8 7 23 77 — 3 5 12 ± 3 (LDH) 13 ± 2  19 ± 2  8 29 70 1 8 4 14 ± 4 (LDH) 14 ± 2  8.7 ± 1  

Example 3.4 Brunauer-Emmett-Teller Data

BET data were collected to obtain surface areas of the more active water oxidation catalysts 5 to 8. See FIG. 11.

Example 3.5 Raman Spectra (FIG. 12)

The Raman spectra of 1 to 3 showed a broad feature centered at around 650 cm⁻¹. In this region, Raman shifts of ferrous-ferric oxides, such as magnetite or ferrihydrite, occur. The broadness observed for 1 to 3, however, strongly suggests the presence of structurally ill-defined, poorly crystallized materials. The Raman spectra of 4 to 8 were compared to a reference spectrum of mineralogical jamborite and showed good agreement. The strong peaks in the spectra of 4 to 8 at ˜1050 cm⁻¹ were assigned to inter-layer nitrate ions, consistent with peaks that have previously been observed in electrochemically deposited α-Ni(OH)₂ thin films. It has been reported that only α-Ni(OH)₂ contained measurable nitrate, as formation of crystalline β-Ni(OH)₂ occurred with the concurrent loss of interstitial layering; the β-polymorph did not accommodate interstitial ions because of tighter crystal packing.

Which Ni(OH)₂ phase is catalytically most active is still subject of intense debate. During water oxidation, α-Ni(OH)₂ is oxidized to γ-NiOOH, whereas β-Ni(OH)₂ is transformed into β-NiOOH; both oxyhydroxides are reduced back to their starting hydroxides during electrochemical cycling. It has been a long-held view that β-Ni(OH)₂ is more active for oxygen evolution. Studies of electrodeposited amorphous α-Ni(OH)₂ and its ageing to β-Ni(OH)₂ in basic electrolytes suggested that oxygen evolution occurred at lower onset potential for β-Ni(OH)₂/β-NiOOH. α-Ni(OH)₂ is known to be highly active for water oxidation.

Example 3.6 Infrared Spectra (FIG. 9)

Infrared (IR) spectra were collected to shed more light on the compositions of catalysts 5 and 6. The IR spectra of 5 and 6 showed broad peaks with maxima at 340, 500, and 640 cm⁻¹. The δ(OH) band at 640 cm⁻¹ is very sensitive to the amount of water intercalated between the α-Ni(OH)2 layers. Bands, attributed to OH-bending motions, typically appear at ˜650 cm⁻¹ for Ni(OH)₂ with high water content and thus indicate the presence of the α-polymorph. In contrast, for the β-polymorph, the band is shifted to ˜520 cm⁻¹. Additionally, the α-polymorph shows broad absorption in the ν(OH) region (3400-3600 cm⁻¹), whereas the β-polymorph features a sharp band at 3640 cm⁻¹. The location and broadness of the δ(OH) and ν(OH) bands in our catalysts 5 and 6 led us to conclude that α-Ni(OH)₂ was the predominant material. The band at 1340 cm⁻¹ was further evidence of interstitial nitrates.

The spectrum of Ni(OH)₂ with iron incorporation was qualitatively determined from published transmission-mode IR spectra. Two materials were used in this analysis, (1) almost exclusively Ni(OH)₂ and (2) one of mixed (Ni,Fe) composition, due to aging in KOH for 72 hours. The compositions of these materials were determined by XRD and Mössbauer spectroscopy in the original study.

The IR spectra were digitized from an electronic (PDF) copy of the original manuscript using UN-SCAN-IT v.5.2 software. Transmission values (digitized γ-values) were aligned with the wavelength (digitized x-values) for both spectra, omitting points where digitization was not complete for both.

The spectrum of (2) was shifted down vertically by assuming that the common feature at 495 nm is isosbestic in transmission. The spectrum of (1) was scaled by a factor consistent with a second isosbestic point at 670 nm. The absorbance spectra of the two samples was then calculated using A(x)=2 log [T(x)], where A(x) is the absorbance and T(x) is the decimal transmission at the wavelength x.

Finally, the spectrum of mixed (Ni,Fe) ‘oxyhydroxide’ was approximated by subtracting the absorbance spectrum of (1) from (2). It is plotted as a red dotted line in FIG. 14, alongside the normalized absorbance spectrum of (1), graphed as a blue dotted line.

It is important to note that, in the absence of an absolute transmission value, these spectra are only qualitative. They do, however, clearly indicate the direction that the peaks shift upon incorporation of iron into the nickel phase. The growth of features at ˜400 cm⁻¹ and ˜600 cm⁻¹ relative to the features at ˜350 cm-1 and ˜650 cm⁻¹ is indicative of iron incorporation into the nickel phase. This trend has been observed previously.

Example 4 Electrochemical Characterization

Electrochemical activity of the nanoparticulate catalysts was assessed by cyclic voltammetry (FIG. 15) and Tafel data (FIG. 16), long-term stability as measured by chronoamperometry. See also FIG. 2.

Chronoamperometry data (FIG. 17) showed that catalytic activity of catalysts 5, 6 and 8 was maintained for more than 5 hours. The current fluctuations were due to formation and release of oxygen bubbles from the electrode surface.

A summary of catalytic activity data is provided in Table 3.

TABLE 3 Overpotentials η at current densities of 0.5 and 10 mA cm⁻², Tafel slopes A, and turnover frequencies (TOF) per gram catalyst at 250 mV and 300 mV overpotential of catalysts 1 to 8 and electrodeposited Ni oxide for comparison. TOF TOF @ η = 250 @ η = 300 η (@ 0.5 η (@ 10 mV mV mA cm⁻²) mA cm⁻²) A (μmol/O₂ (μmol/O₂ Catalyst (mV) (mV) (mV/dec) s⁻¹ g⁻¹) s⁻¹ g⁻¹) 1 360 520 84.7 ± 2.1  0.23 0.89 2 300 470 73.3 ± 1.0  0.94 4.6 3 240 300 48.7 ± 0.7  7.1 78 4 230 290 47.5 ± 1.3  11 130 5 220 280 47.6 ± 0.6  21 220 6 220 350 42.0 ± 0.9  19 42  190 ± 11.6 7 210 270 45.2 ± 0.7  33 290  139 ± 35.6 8 200 260 44.7 ± 2.0  53 290  294 ± 90.6 Ni oxide 280 370 41.5 ± 0.6  0.63 10  170 ± 52.0

A comparison with published Fe—Ni-based water oxidation catalysts is provided in Table 4. Direct comparability of catalytic activity is in general problematic because of variations in mass loading, film thickness, intricate details of the electrochemical measurements, such as electrode substrate, rotation speed and dwell time to reach steady-state conditions or scan rates; also, overpotentials were recorded at different current densities. Nevertheless, we compiled published data and compared them with our catalysts made by PLAL. When measured at a current density of 10 mA cm⁻² on a flat electrode substrate, our best catalyst had the lowest overpotential.

TABLE 4 Comparison of overpotentials η (at given nominal current densities) of this work with reported catalysts. Electrode substrate materials are also given because only flat working electrode substrates allow for a meaningful comparison of electrocatalyst performance. Electrode Current density η Reference Catalyst substrate (mA cm⁻²) (mV) (below) 8 Flat HOPG 10 260 this work 5 Flat HOPG 10 280 this work Thin-film solution- Au/Ti- 10 336 (i) cast Ni_(0.9)Fe_(0.1)O_(x) coated quartz crystal Nanostructured α- Glassy 10 331 (ii) Ni(OH)₂ carbon Electrodeposited Glassy 10 360 (iii) NiFeOx carbon Thin-film Gold 10 280 (iv) electrodeposited Ni-Fe (40% Fe) Graphene FeNi Ni foam, 10 220 (v) double hydroxide unspecified hybrid pore size* Thin film nickel oxide Nickel foil 8 230 (vi) with iron impurities Ni-Fe layered double Carbon 5 290 (vii) hydroxide nanoplates fiber paper β-NiOOH Nickel, 5 500 (viii) polished with μm- sized alumina powders Mixed Fe-Ni oxides Carbon 1 375 (ix) paper Nickel-borate Glassy 1 425 (x) carbon Amorphous α- FTO glass 0.5 210 (xi) Fe₂₀Ni₈₀O_(x) High surface-area Nickel 0.5 265 (xii) nickel metal oxides or steel microdiscs NiFeAlO₄ inverse Glassy 0.1 380 (xiii) spinel carbon NiO_(x) deposited from Glassy 0.1 390 (xiv) molecular [Ni(en)₃]²⁺ carbon *The high porosity of nickel foam leads to an enlargement of the electrode substrate surface area relative to the apparent geometric area, inflating current densities that are normalized to the geometric electrode area. (i) Trotochaud, L. et al., J. Am. Chem. Soc. 2012, 134, 17253; (ii) Gao, M. et al., J. Am. Chem. Soc. 2014, 136, 7077. (iii) McCrory, C. C. L. et al., J. Am. Chem. Soc. 2013, 135, 16977. (iv) Louie, M. W., et al., A. T. J. Am. Chem. Soc. 2013, 135, 12329 (v) Long, X. et al., Angew. Chem., Int. Ed. Engl. 2014, 53, 7584. (vi) Corrigan, D. A. J. Electrochem. Soc. 1987, 134, 377. (vii) Gong, M et al., J. Am. Chem. Soc. 2013, 135, 8452. (viii) Yeo, B. S., et al., Phys. Chem. C 2012, 116, 8394. (ix) Landon, J. et al., ACS Catal. 2012, 2, 1793. (x) Dinc{hacek over (a)}, M. et al., Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10337. (xi) Smith, R. D. L. et al., J. Am. Chem. Soc. 2013, 135, 11580. (xii) Li, Y.-F.; Selloni, A, ACS Catal. 2014, 4, 1148. (xiii) Chen, J. Y. C. et al., Energy Environ. Sci. 2014, 7, 1382. (xiv) Singh, A., et al., L. Energy Environ. Sci. 2013, 6, 579.

The following additional references may also be helpful in understanding certain elements of the present invention:

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As those skilled in the art will appreciate, numerous modifications and variations of the present invention are possible in light of these teachings, and all such are contemplated hereby. For example, in addition to the embodiments described herein, the present invention contemplates and claims those inventions resulting from the combination of features of the invention cited herein and those of the cited prior art references which complement the features of the present invention. Similarly, it will be appreciated that any described material, feature, or article may be used in combination with any other material, feature, or article, and such combinations are considered within the scope of this invention.

The disclosures of each patent, patent application, and publication cited or described in this document are hereby incorporated herein by reference, each in its entirety, for all purposes. The Appendices to this description are likewise considered part of this application.

The file of this patent or application contains at least one drawing/photograph executed in color. Copies of this patent or patent application publication with color drawing(s)/photograph(s) will be provided by the office upon request and payment of the necessary fee. 

What is claimed:
 1. A layered double hydroxide nanostack comprising a plurality of nanosheets represented by the general formula: [M_((1-x))M′_(x) (OH)₂]^(x+), wherein: M is a metal cation in a formal +2 oxidation state; M′ a metal cation in a formal +3 oxidation state; wherein the nanosheets are associated with or intercalate [A^(m−) _(x/m)], where A is a displaceable anion; m is an integer; x is a positive number less than 1; the nanostack being optionally hydrated with a stoichiometric amount or a non-stoichiometric amount of water; and the nanosheet having at least one lateral edge dimension in a range of from about 5 nm to about 100 nm.
 2. The layered double hydroxide nanostack of claim 1, wherein M is at least one of Ba²⁺, Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺, and Zn²⁺, and M′ is at least of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, Y³⁺, Ce³⁺.
 3. The layered double hydroxide nanostack of claim 1, wherein A comprises an organic or inorganic anion, or a combination thereof.
 4. The layered double hydroxide nanostack of claim 3, wherein A comprises F⁻, Cl⁻, Br⁻, I⁻, BF₄ ⁻, PF₆ ⁻, CO₃ ²⁻, HCO₃ ⁻, CrO₄ ²⁻, NO₂, NO₃, ONO₂ ⁻, ClO₂ ⁻, ClO₃ ⁻, ClO₄.⁻, H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, IO₃ ⁻, OH⁻, S²⁻, SO₃ ²⁻, S₂O₃ ²⁻, SO₄ ²⁻, WO₄ ²⁻, or a combination thereof.
 5. The layered double hydroxide nanostack of claim 3, wherein A comprises acetate, propionate, lactate, terephthalate, adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a combination thereof.
 6. The layered double hydroxide nanostack of claim 3, wherein A comprises a complex anion comprising a transition metal compound.
 7. The layered double hydroxide nanostack of claim 1, wherein M is or comprises Ni²⁺.
 8. The layered double hydroxide nanostack of claim 1, wherein M′ is or comprises Fe³⁺.
 9. The layered double hydroxide nanostack of claim 1, wherein x is in a range of from 0.05 to 0.95.
 10. The layered double hydroxide nanostack of claim 1, wherein A is or comprises hydroxide.
 11. The layered double hydroxide nanostack of claim 1, wherein the nanosheet has at least one lateral edge dimension in a range of from about 7 nm to about 22 nm.
 12. The layered double hydroxide nanostack of claim 1, wherein at least one of the nanosheets is further doped with at least one Lewis acid ion of a transition metal, lanthanide, or actinide metal ion.
 13. The layered double hydroxide nanostack of claim 1, wherein at least one of the nanosheets is further doped with titanium or lanthanum.
 14. A method of preparing a layered double hydroxide nanostack of claim 1, comprising subjecting a solid ablation target to an energy source, the solid ablation target comprising a first metal capable of oxidizing to a positive oxidation state, the energy source impinging on the first metal in the presence of an aqueous ablation solution containing a second metal ion in a positive oxidation state for a time and energy sufficient to ionize at least a portion of the first metal to a positive oxidation state.
 15. The method of claim 14, wherein the aqueous ablation solution is substantially free of surfactants.
 16. The method of claim 14, wherein the first metal is capable of achieving a +2 oxidation state, and the second metal is in a +3 oxidation state.
 17. The method of claim 14, wherein the first metal is capable of achieving a +3 oxidation state, and the second metal is in a +2 oxidation state.
 18. The method of claim 16, wherein the first metal comprises beryllium, calcium, cadmium, copper, cobalt, iron, magnesium, manganese, nickel, strontium, zinc, or an alloy or mixture thereof and the second metal ion comprises at least one of Al³⁺, Ce³⁺, Co³⁺, Cr³⁺, Fe³⁺, Ga³⁺, In³⁺, La³⁺, Mn³⁺, V³⁺, and Y³⁺.
 19. The method of claim 17, wherein the first metal comprises aluminum, cerium, cobalt, chromium, iron, gallium, indium, lanthanum, manganese, vanadium, yttrium, or an alloy or mixture thereof, and the second metal comprises at least one of Be²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Co²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺, and Zn²⁺.
 20. The method of claim 14, wherein the energy is a laser.
 21. The method of claim 20, wherein the laser energy is provided in pulses
 22. The method of claim 21, wherein the laser pulse delivers an energy in a range of from about 90 mJ/pulse to about 210 mJ/pulse.
 23. The method of claim 14, wherein the displaceable ion A of the layered double hydroxide nanostack comprises a counterion associated with the second metal ion in the aqueous ablation solution.
 24. The method of claim 23, further comprising exchanging the displaceable ion A of the layered double hydroxide nanostack with a different anion.
 25. The method of claim 14, wherein the initially-formed layered double hydroxide nanostack is further sintered.
 26. An electrode comprising a coating comprising the layered double hydroxide nanostack of claim
 1. 27. The electrode of claim 26, wherein the electrode comprises gold, nickel, platinum, or an allotrope of carbon.
 28. The electrode of claim 26, wherein the electrode comprises graphite, graphene, glassy (or vitreous) carbon, diamond, or a combination thereof.
 29. The electrode of claim 27, that exhibits an overpotential for the oxidation of water to oxygen of less than 300 mV at 10 mA/cm² on a flat supporting electrode.
 30. An electrochemical cell comprising an electrode of claim
 26. 31. A method for oxidizing water comprising applying a potential to an electrode of claim 26, and passing sufficient current to oxidize water to form oxygen.
 32. The method of claim 31, wherein the potential is within 300 mV of the thermodynamically determined potential for the oxidation of water to oxygen at 10 mA/cm². 